1. Field of the Invention
This invention relates to a system for compensating for variations in an output, and more particularly to an output compensating system for compensating for variations in an output which consists essentially of a plurality of components showing variations proportional to the magnitude of inputs.
2. Description of the Prior Art
For example, in a solid state image pickup device comprising many light sensing elements, such as a CCD image sensor or an MOS image sensor, the light sensing elements exhibit somewhat non-uniform sensitivity due to variations in their light response characteristics, transfer efficiency, etc. Such non-uniformity in the sensitivity of the sensing elements results in an image sensor output comprising a plurality of components showing variations as mentioned above. Examples of the output voltages of a CCD line image sensor with reference to incident light having a uniform level of illumination are shown in the graph of FIG. 1. In this graph, the output voltage is plotted on the ordinate, and the positions of the light sensing elements on the abscissa. Variations in output voltage at any given level of incident light intensity indicated variations in sensitivity. It will also be noted that the magnitude of these variations differs with the level of the incident light intensity.
From an examination of the non-uniformity of the output voltages (variations in the sensitivity) shown in FIG. 1 for the light sensing elements, it is found that the width of fluctuation in the output voltage is approximately proportional to the level of the mean output voltage. It is also noted that the pattern of this fluctuation is constant regardless of the level of the mean output voltage. That is, the output voltage can be expressed by the following equation: EQU A(n)=.alpha.(n).multidot.B(n) n=1, 2, . . . , (1)
in which A(n) is the output voltage of the n-th light sensing element, .alpha.(n) is the output voltage regulation of said light sensing element, and B(n) is the mean output voltage (ideal output voltage) of said light sensing element.
From Equation (1), it can be said that the non-uniformity of the output voltage of the solid state image pickup device (variation in the sensitivity) is a fixed pattern noise which is represented by a product with reference to the output voltage.
In image reading units using solid state image pickup devices, e.g. TV cameras, telecines and facsimile machines, the aforesaid variations in the sensitivity cause a fixed pattern noise superposed on the output voltage, bringing about an adverse effect.
As a method for eliminating the fixed pattern noise due to the sensitivity variations of the light sensing elements of such a solid state image pickup device, it is well known to store a fixed pattern for each light sensing element in a ROM (Read Only Memory). The signal of the corresponding fixed pattern noise value is read out synchronously with each component of the output signal of the solid state image pickup device. Then, the aforesaid output signal is divided by this read-out signal. In this method, the following calculation is made through a combination of the above-mentioned ROM and a divider circuit: EQU B(n)=A(n)/.alpha.(n) (2)
In order to carry out this method, however, it is necessary to use a ROM of a large capacity corresponding to the number of light sensing elements of the solid state image pickup device and to the required gradation levels, as well as a high-speed high-accuracy divider circuit. For example, when such a device has 1024 light sensing elements and 8 bits of gradation, the ROM must have a capacity as large as about 8 kilobits. Thus this method has not been put into practical use for economical reasons.